1. Field of the Invention
Modem heating pads and electronic blankets have heater wires that do not require separate thermostats. They fall into two basic types: a heater wire having a PTC heating layer disposed between two conductors that has an increased resistance with temperature increase so that the wire is self limiting and not subject to hot spots; or a heater wire that provides a feedback signal to a control for monitoring temperature and detecting local hot spots. This subject invention relates to the latter, having a feedback signal that is both responsive to the temperature of the wire and has increased sensitivity when portions of the wire are at a temperature that is higher than the average temperature.
2. Description of the Prior Art
The first successful invention that uses a feedback signal for temperature control concurrently with a voltage that also indicates the occurrence of a hot spot that deteriorates the insulation between a heater conductor and a sensor wire was invented by this inventor and is described in U.S. Pat. No. 5,861,610. A Positive Temperature Coefficient (PTC) Nickel alloy sensor wire is counter wound around a heater wire with an inner insulation therebetween. The current leakage through the insulation provides an electrical couple between the sensor and heater wire. The resistance of the sensor wire is measured and used for temperature control. An AC voltage present on the sensor wire indicates the existence of a breakdown in the separating insulation. Where Poly Vinyl Chloride (PVC) is used as the separating layer, small leakage would occur at about 160° C. Where polyethylene is used, the layer melts at 130° C. and contact is made between the heater and sensor wire. Either case would signal the control unit to disconnect the power to the heater conductor.
A similar technique was patented by Gerrard and is disclosed in U.S. Pat. No. 6,310,322, who used the second conductor as a heater with the insulation between having an enhanced NTC (Negative Temperature Coefficient) characteristic. The two heating conductors are connected through a diode so that leakage through the NTC layer introduces the negative half cycle, which presence is caused to terminate the power. In a second embodiment, the second conductor is a PTC sensor wire, such as disclosed in the aforementioned U.S. Pat. No. 5,861,610.
A smaller more flexible heater wire design is taught by Barry Keane in U.S. Pat. No. 6,222,162 that uses a single conductor of a PTC alloy for both heating and temperature sensing. In the Keene device, no attempt is made to detect local hot spots that occur from bunching or folding. Only the average temperature is used to control the wire temperature.
All of the above references measuring resistances within the wire by voltage comparison techniques. The resistance changes are generally small and much attention is paid to tolerances in the application of these techniques in production.